The M1 HPA pure Class A headphone amp's technical performance is outstanding. Distortion is incredibly low (less than .0005% across the band) frequency response is ruler flat. Noise ratio is outstanding. In every technical parameter the M1 pure Class A headphone amp is outstanding.
The HPA has very low output impedance (below 1 ohm). Headphone amps' output impedance is a usually ignored but very important measurement. The HPA's extraordinary low output impedance means that it can drive any headphone with complete linearity and low distortion. The vast majority of competitive headphone amplifiers cannot do this because they have much higher output impedance.